METHOD FOR ADJUSTING TRANSMISSION POWER IN A RADIO SYSTEM AND CORRESPONDING RADIO SYSTEM

Abstract

A radio system wherein transmission power is adjusted by means of a receiver (1; 2) which evaluates the signal from a transmitter (2; 1) and determines output-related adjustment information (TPC) which is sent to the transmitter (2; 1) during successive time slots (4) in order to adjust said transmission power. The adjustment information (TPC) is transferred to the transmitter (2; 1) in several successive time slots (4) in a slotted mode after a section (9) which is not filled with information .

Full Text

Method for controlling the transmission power in a radio system, and a corresponding radio oyctom
The present invention relates to a method as claimed in the precharacterizing clause of claim 1 for controlling the transmission power in a radio system, and to a corresponding radio system as claimed in the precharacterizing clause of claim 12, in particular to a corresponding mobile radio system.
Control of the transmission power represents an important performance feature in mobile radio systems, in order to suppress possible interference between the individual connections, and thus to make it possible to improve the capacity and quality of the connections, in order to be able to reduce the mean transmission power and to be able to match it as well as possible to the requirements, as well as to make it possible to compensate, at least partially, for losses in the transmission channels.
For this purpose, the signal transmitted by a transmitter is evaluated at the receiving end in the mobile radio system, in order to make it possible to this to produce information for power control and to transmit this to the transmitter, which then adjusts the transmission power in accordance with the power control or power adjustment information.
In this case, the power adjustment information is transmitted analogously to the transmission of the actual communication information, depending on the
respective mobile radio system, embedded in a predetermined frame and timeslot structure, that is to say the information is transmitted in a number of sequentially transmitted frames, with each frame having a specific number of timeslots. For known mobile radio systems., it has been proposed for the transmitter "of the mobile radio system to be operated in a mode which is referred to as the slotted mode or compressed mode, with the information to be transmitted in this case being transmitted within specific frames in -
compressed fc rm to a receiver in order to make it possible, in the appropriate frame, to produce a section which is free of information bits, and which is referred to a an idle slot, which can then be used for intermediate frequency measurements, for example in order to prepare for a handover between different mobile radio systems, The information must be transmitted in a shorter time . interval, for compression.
The principle of compression is illustrated schematically in Figure 4, with a number of sequentially transmitted frames 3 being shown, each of which has an identical frame duration, for example 10 ms. The second frame 3 shown in Figure 4 uses the slotted mode, that is to say the information is transmitted in compressed form in this frame, so that an idle slot 9 occurs, in which now information is transmitted. As is likewise shown in Figure 4, the transmission power can be increased during this frame 3 that is operated in the slotted mode, in order to achieve a transmission quality which is not adversely affected by the slotted mode.
However, the slotted mode interrupts the principle of fast power control. In modern mobile radio systems, power adjustment information for the transmitter is produced in each timeslot, so that the transmission power can be adapted relatively quickly. However, no such power ad3ustment information can be transmitted during the idle slots described above. Firstly, this results in a higher error rate for the power adjustment information bits and, secondly, it must be expected
that the instantaneous transmission power will deviate to a greater extent from the nominal value.
In order to overcome this problem associated with the slotted mode, it has been proposed, for example, to temporally increase the energy in what are referred to as pilot bits after the slotted mode,
for power control, in order to allow correct decoding of the power adjustment information, which is generally formed only by a corresponding bit with a specific mathematical sign. The pilot bits are used to estimate the channel impulse response during what is referred to as a training sequence, and correspond to a known bit pattern. The power control procedure for the slotted mode as described in this document is, however, relatively complicated, and requires a relatively large amount of effort.
The present invention is thus based on the object of proposing an improved method for controlling the transmission power in a radio system, and of proposing a corresponding radio system, in which case the aim is to reliably control the transmission power in particular even in the slotted mode or compressed mode described above.
According to the invention, this object is achieved by a method having the features of claim 1, and by a radio system having the features of claim 12. The depending claims each define preferred embodiments of the present invention.
The invention proposes that the same power adjustment information be transmitted in successive timeslots, so that the transmission power in the transmitter can be set with greater reliability, by evaluating the power adjustment information received during these timeslots in combination in order to adjust the transmission power.
The invention is parti :ularly suitable for use in what is referred to as the slotted mode, in which case the same power adjustment information is transmitted a number of times following an idle slot, that is to say a section of a frame in which no information is transmitted. However, the invention is not restricted to this use in the slotted mode, that is to say
the invention can in principle also be used for timeslots which do not follow an idle slot, in order to allow more reliable power control in these situations as well. In this case, this approach is of particular interest when the transmission conditions in the respective transmission channel are not changing rapidly (for example when a mobile station is moving at slow speed).
In particular, the invention proposes that the signal-to-noise ratio of the received signal be evaluated, for example, after an idle slot, that appropriate power adjustment information be produced as a function of this, and that this information be transmitted to the transmitter during the next two timeslots immediately following the idle slot. The transmitter then evaluates the power adjustment information received during these two timeslots and adjusts the transmission power taking account of both power adjustment information items, so that a transmission power adjustment which may have been made on the basis of the first power adjustment information item can still be corrected.
The principle on which the present invention is based can be used both in the uplink, that is to say for transmitting communication information from a mobile station to a base station, and in the downlink, that is to say for the opposite direction, or else for both directions at the same time.
After receiving the power adjustment information in the first timeslot immediately following the idle slot, the transmission power can initially be kept constant, so
that no change is made until the power adjustment information in the second timeslot has been received.
Generally, the power adjustment information transmitted during each timeslot comprises only one bit, with the step width for changing the transmission power
being coded digitally. However, instead of this, the
step width can also be coded in analogue form, that is to say the step width then cepends, for example, directly analogously on the discrepancy between the received signal level measured in the receiver and a predetermined reference value.
The invention requires no change whatsoever to the specified timeslot format. The combination of the successively transmitted power adjustment information items results in an improvement in the available bit-signal-noise ratio (Eb/No) . This increased bit-signal-noise ratio allows a greater step width to be provided for power control without any risk of an increased bit error rate more frequently resulting in the transmission power needing to be changed in the wrong direction.
The invention can be used in various types of radio system, although the present invention is of particular interest for mobile radio systems using code division multiplexing methods (code division multiple access CDMA).
The invention will be explained in more detail in the following text with reference to the drawing, based on preferred exemplary embodiments.
Figure 1 shows an illustration to explain the principle on which the invention is based, on the basis of a preferred exemplary embodiment,
Figure 2 shows a general schematic illustration to explain the information transmission in a mobile radio system,
Figure 3 shows the frame and timeslot structure for what is referred to as a downlink connection, according to the present standard of UMTS Standardization, and
Figure 4 shows an illustration to explain the frame structure in what is referred to as the slotted mode.
According to the present invention there is provided a radio system, comprising :
a transmitter,
a receiver configured to receive a signal transmitted via a transmission channel in the radio system from the transmitter, and to evaluate the received signal in order to produce power adjustment information as a function of it, and to send this information to the transmitter, with the transmitter being designed to adjust the transmission power as a function of the power adjustment information from the receiver, with information being transmitted, embedded in a frame and a timeslot structure, between the transmitter and the receiver, with a signal, wherein the receiver is designed to transmit identical power adjustment information to the transmitter in a number of successive timeslots.
The principle of power control will first of all be explained in more detail with reference to Figure 2, with Figure 2 illustrating the communication between a base station 1 and a mobile station 2 in a mobile radio system. A connection from the base station 1 to the mobile part 2 is referred to as a downlink or forward link, while a connection from the mobile part 2 to the base station 1 is referred to as an uplink or reverse link. For downlink power control, the respective received signal is evaluated in the mobile station 2, and power adjustment information or power control information is produced as a function of it, and is sent back to the base station 1, so that the base station 1 can set the transmission power as appropriate. For uplink control, the received signal is evaluated in the base station 1, where the power control information is produced, and the mobile station 2 is instructed to carry out power adaptation.
The power control information is in this case
transmitted embedded in a predetermined frame
structure, depending on the respective mobile radio
system.
As an example, Figure 3 shows the frame ..and timeslot structure for a downlink connection in a mobile radio system which is operated using a code division multiple access metho_d_XCDMA) . The frame and timeslot structure shown in Figure 3 corresponds in particular to a UMTS mobile radio channel (Universal Mobile Telecommunications System), which is also referred to
as a DPCH (Dedicated Physical CHannel), in accordance with the current state of UMTS Standardisation. UMTS is the designation for third-generation mobile radio systems, aiming to provide a worldwide, universal mobile radio standard. According to the UMTS mobile radio standard, the multiple access method is what is referred to as the .
WCDMA method (Wideband Code Division Multiple Access).
The frame structure shown in Figure 3 has a duration of 720 ms and comprises, in particular, 72 identically constructed frames 3 with a frame duration of 10 ms, with each frame in turn having 16 timeslots 4, each with a timeslot duration of 0.625 ms. Alternatively, a frame 3 may also comprise 15 correspondingly longer timeslots 4. However, the former situation is assumed in the following text. Each timeslot 4 comprises information which is split between a logical control channel (DPCCH, Dedicated Physical Control Channel) and a logical data channel (DPDCH, Dedicated Physical Data Channel). The DPCCH section comprises a pilot bit sequence 5 and what is referred to a TPC information (Transmitter Power Control) 6 and TFI information (Transmitter Format Identifier) 7. The DPDCH section comprises user data bits 8.
The pilot bit sequence 5 is used for estimating the channel impulse response during what is referred to as a training sequence, and corresponds to a known bit pattern. The receiver can determine or estimate the channel impulse response of the mobile radio channel by comparing the received signal with the known pilot bit sequence.
The TFI information 7 is used for format identification for the respective receiver. The TFI bits are protected by means of their own coding method in accordance with the present WCDMA standard, and are distributed over an entire frame (time period 10 ms) by interleaving. If, for example, the TFI information 7 in each timeslot comprises two bits, then this results in a total of
2*16 = 32 TFI bits per frame, which comprises 16 timeslots, and these are coded by means of what is referred to as a biorthogonal coding method.
The TPC information 6 represents the command, produced by the receiver and transmitted to the transmitter, to adjust the
transmission power. For this purpose, the received power or the signal-to-noise ratio of the received signal is compared with a predetermined reference value in the receiver, and the value for the power adjustment command is determined as a function of the discrepancy. This means that, if the received power exceeds the reference value, a command is produced to reduce the transmission power, while a command to increase the transmission power is produced if the received power is less than the predetermined reference value. Thus, depending on the comparison result, the receiver thus transmits a digital or binary adjustment command to the transmitter. In this case, a command to increase the transmission power (power up command) is coded by a 1, while a command to reduce the transmission power (power down command) is coded by a zero. In each case, the adjustment command is transmitted to the transmitter after appropriate modulation. In accordance with the currently discussed WDCMA Standard for UMTS mobile radio systems, the transmission is carried out by means of QPSK modulation (Quadrature Phase Shift Keying), as a result of which the binary 1 or 0, respectively, is mapped onto the value -1 or +1, respectively, with subsequent spreading of the power control signal.
The power adjustment or power control information thus generally comprises only one bit, which indicates whether the transmission power should be increased or decreased at the transmission end. In order to allow this bit to be transmitted with a sufficiently low error probability, the bit is transmitted repeatedly within the TPC field 6. The TPC information shown in Figure 3 in consequence comprises, for example, the
three bits which are transmitted successively with identical information content.
However, despite the repeated transmission of the TPC bits within the TPC field 6, the error rate is increased by the previously described slotted mode or compressed mode. In order to counteract this, before [lacuna]
information 6 to be transmitted repeatedly, with this being done especially during the two timeslots 4 which immediately follow the idle slot 9.
In order to explain this principle, Figure 1 shows, by way of example, the structure of the already previously mentioned DPCCH control channel (see Figure 3) in a frame, assuming that the timeslots #n shown in Figure 1, is the timeslot immediately following is an idle slot, that is to say a section which is not filled with information. The profile of the transmission power, which is set in the transmitter as a function of the respectively transmitted TPC information, is shown underneath the timeslot structure.
In the receiver, the signal-to-noise ratio of the signal being received from the transmitter at that time is measured in order to determine the TPC or power adjustment information to be transmitted during the timeslot #n, is compared with a reference value and is used to produce the TPC information TPCn- This TPC information is transmitted to the transmitter during the timeslot #n, where it is decoded and used to adjust the transmission power. In this case, it is possible to wait first of all in the transmitter for reception of the TPC information TPCn+1 transmitted during the next timeslot #n+l, before finally setting the transmission power to the desired value, so that it is recommended that the transmission power initially be set in the transmitter in accordance with a "normal" nominal step width, which corresponds to a normal Eb/No ratio, after receiving the TPC information TPCn. This step width is indicated by APn in Figure 1, and corresponds to the difference between the instantaneous
transmission power Pn-1 and the transmission power Pn set after reception of the TPC information TPCn. The value 0 may also be chosen as the step width APn after reception of the power adjustment information TPCn, in order to keep the transmission power constant until the power adjustment information TPCn+1 is received, since it is then possible to make a decision.
on the transmission power to be set, with greater reliability.
In order to produce the TPC information to be transmitted during the next timeslot #n+l, the TPC information from the previous timeslot is simply repeated, that is to say TPCn+1 = TPCn, rather than evaluating the signal-to-noise ratio of the transmission signal once again. In this way, the ratio Eb/No can be increased since, after receiving the timeslot #n+l, the TPC information TPCn+i can be combined with the already previously received TPC information TPCn, and the transmitter is thus able to set the transmission power to the desired value with greater reliability. If, for example, the transmitter identifies the fact that the decision made as a consequence of the most recently received TPC information was incorrect, this can now be corrected. As shown in Figure 1, the transmitter can now also change the transmission power with a larger step width, thus reducing the risk of a further deterioration being caused after an incorrect TPC decision. This is indicated in Figure 1 by APn+i, with both APn and APn+i indicating the step width with respect to the transmission power Pn-1 set after the idle slot.
The principle, mentioned above, of repeated transmission of the same TPC information during successive timeslots can be applied not only to the first time after an interruption in the power control by the idle slot, but also during a normal connection. In this case, this is of particular interest for those situations in which the transmission channel does not change so fast with time, since the disadvantage,
associated with the method, of an increased delay in the power control can then be more than compensated for by the advantage of better quality of the transmitted power adjustment information. Since the smallest step width for power control is currently 1 dB and, for implementation reasons, smaller
values are not suitable for use in mobile stations, this provides a possible way to reduce the power control rate.
Furthermore, the invention can be used to transmit the power adjustment information with a smaller number of TPC bits. Although this reduces the bit-signal-noise ratio (Eb/No) , this can be compensated for, however, by the advantage of the invention, namely by increased transmission reliability. The use of a small number of TPC bits is advantageous to the extent that this reduces what is referred to as the overhead, and the efficiency can thus be improved.
Furthermore, the present invention can also advantageously be used for what is referred to as a soft handover. In this case, the mobile station 2 transmits and receives data from and to a number of base stations 1. To detect the user data, the received signals from all the connections are combined, thus allowing the power of each individual connection to be reduced. The TPC power adjustment commands may, however, in general not be combined, since the TPC power adjustment commands sent from different base stations 1 (via a downlink) need not be identical. Since the TPC power adjustment commands must also be evaluated very quickly, it is also generally impossible to combine them in the uplink. Otherwise, intolerable delays would occur, caused by the corresponding information being passed on from one base station 1 to another base station 1. In addition to increasing the power level of the number of TPC bits, this could be overcome by use of the present invention.
Finally, the previously described invention can also be combined with the principle of analogue transmission of the step width for adjustment of the transmission power. In this case, the transmission power to be set in the transmitter and the ,
corresponding step width are coded in analogue form rather than digitally, that is to say the power adjustment information is in each case set in analogue form depending on the discrepancy between the measured received signal level and the predetermined reference value, for example the pilot bit reception level. A discrepancy that is twice as great between the received signal level and the reference value thus results in a step width which is twice as large.

WE CLAIMS:-
1. A radio system, comprising
a transmitter (2; 1),
a receiver (1; 2} configured to receive a signal transmitted via a transmission channel in the radio system from the transmitter
(2; 1), and to evaluate the received signal in order to produce power adjustment information (6) as a function of it, and to send this information (6) to the transmitter (2; 1), with the transmitter (2; 1) being designed to adjust the transmission power as a function of the power adjustment information from the receiver (1; 2), with information being transmitted, embedded in a frame and a timeslot structure (3, 4), between the transmitter
(2; 1) and the receiver (1; 2), with a signal, wherein
the receiver (1; 2) is designed to transmit identical power adjustment information (6) to the transmitter (2; 1) in a number of successive timeslots (4).
2. The radio system as claimed in claim 1, wherein the
information being transmitted is in compressed form in a specific
frame (3) , so that there is a section (9) which is not filled
with information within this frame (3), the receiver (1; 2) being
designed to transmit identical power information (6} to the
transmitter (2; 1) in a number of successive timeslots (4)
following a section (9) which is not filled with information in a
frame (3).
3. The radio system as claimed in claim 2, wherein the receiver
(1; 2) is designed to transmit identical power adjustment

information (6) to the transmitter (2; 1) in two successive timeslots (4) following the section (9) which is not filled with information.
4 . The radio system as claimed in claim 3 , wherein the transmitter (2; 1) is designed to adjust the transmission power as a function of both power adjustment information (6) received in the first timeslot (4) following the section which is not filled with information, and power adjustment information (6) received in the second section (9) following the section (9) not filled with information.
5. The radio system as claimed in claim 3 or 4, wherein the
receiver (1; 2) is designed to once again evaluate the signal,
received by the receiver (1; 2) from the transmitter (2; 1) after
the transmission of the identical power adjustment information
(6) in the first and second timeslots (4) following the section
(9) which is not filled with information, and to produce new
power adjustment information as a function of this, and to send this information to the transmitter (2; 1) in the next timeslot (4) .
6. The radio system as claimed in any of the preceding claims
wherein the transmitter (2; 1) is designed to change the
transmission power by a fixed amount ( Pn) after receiving the
power adjustment information transmitted in the first timeslot
(4) following the section (9) which is not filled with information and, after receiving the power adjustment information (6) received during the second timeslot (4) following the section (9) which is not filled with information, and, additionally to
take account of the power adjustment information (6) received during the first timeslot (4) following the section (9) which is not filled with information, to determine a power change value (Pn+1) , and to change the transmission power in the transmitter (2; 1) in accordance with this power change value ( Pn+1)•
7. The radio system as claimed in claim 6, wherein the fixed amount ( Pn) corresponds to the value zero, so that after receiving the power adjustment information transmitted in the first timeslot (4) following the section (9) which is not filled with information, the transmitter (2; 1) is configured to transmit constant power until reception of the power adjustment information (6) transmitted in the second timeslot (4) following the section (9) which is not filled with information.
8. The radio system as claimed in one of the preceding claims wherein the receiver (1; 2) is designed to produce the power adjustment information (6) analogously as a function of the discrepancy between a specific evaluated parameter in the received signal and a corresponding reference value.
9. The radio system as claimed in one of the preceding claims, wherein the receiver (1; 2) is designed to also transmit identical power adjustment information (6) to the transmitter (2; 1) in a number of successive timeslcts (4) which do not directly follow a section (9) which is not filled with information, and in that the transmitter (2; 1) is designed to adjust the transmission power taking account of the power adjustment information received during these timeslots (4).
10. The radio system as claimed in one of the preceding claims,
wherein the radio system is a CDMA mobile radio system.
11. A radio system, substantially as hereinbefore described
with reference to the accompanying drawings.